Focus correction systems and methods are aimed to minimize (and even reduce to zero) focus errors. Light based focus error correction systems illuminate an inspected object, receive light reflected from the inspected object and determine a focus error based upon the spatial relationship between images that are imaged on an imaging sensor. The spatial relationship is translated to a focus error correction signal that is fed to a motor that can mechanically move the inspected object such as to ideally minimize (and even cancel) the focus error.
Due to the high accuracy required from the motor, as well as due to mechanical limitations, the motor is characterized by a relatively long response period. This long response period can well exceed the scanning period of height differentiated narrow features of the inspected object. Accordingly, a focus, error correction unit is not able to properly compensate for height differences between the height differentiated narrow features and their surroundings.
In addition, many inspected objects (including many integrated circuits) include a relatively transparent layer (such as a Copper Mechanical Polish layer) that is positioned above a highly reflective layer (such as a copper layer). When such inspected objects are illuminated with a light beam, most of the reflected light will originate from the highly reflective layer and the focus error signal will mainly represent the upper surface of the highly reflective layer and not the upper surface of the inspected object. By minimizing (and even canceling) the focus error signal the focus error correction unit focuses on the highly reflective layer and the surface of the inspected object will be out of focus.
There is a need to provide efficient methods for focus error corrections and efficient systems having focus error correction capabilities.